Composite scintillators

ABSTRACT

Embodiments of composite scintillators which may include a scintillator material encapsulated in a plastic matrix material and their methods of use are described.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/934,016, filed Jan. 31, 2014, which is incorporated herein byreference in its entirety.

FIELD

Disclosed embodiments are related to composite scintillators.

BACKGROUND

Since the availability of ³He is becoming scarce, it is crucial todevelop new sensors, which are capable of detecting neutrons with highsensitivity. In addition, the potential dual mode of operation affordedby the simultaneous detection of gamma rays and neutrons with a singlescintillator will dramatically reduce the complexity of the instruments,which will provide for their wide deployment. In order to achieve a lowfalse detection alarm rate, the main detector requirements include highefficiency, good energy resolution, and the ability to differentiatebetween gamma and neutron events. While a number of new materials havebeen identified and are being developed at present, they are oftenexpensive and not readily available. Efforts need to be undertaken tofind better and less expensive materials for γ-ray and neutrondetection.

Radiation Monitoring Devices, Inc. (“RMD”) has invented a number ofscintillator materials with improved performance over classicalmaterials. Some of these materials offer thermal and fast neutrondetection simultaneously with gamma ray detection, providing multimodeoperation. One of the first such materials is Cs₂LiYCl₆ (CLYC), whichoffers (1) improved energy resolution of better than 4% at 662 keV; (2)efficient thermal neutron detection (2×higher than ³He at 10atmospheres); and (3) excellent separation between gamma and neutronparticles (>10⁻⁶). CLYC is already in a stage of intensivecommercialization with 1″ and 2″ inch crystals becoming standardproducts. FIG. 1A shows 1″ and 2″ CLYC packaged detectors produced andoffered by RMD. Detection instruments and systems based on CLYC arebeing developed internally and by a number of companies and nationallabs. As an example, in FIG. 1B is shown the Thermo-Scientific'scommercial gamma-neutron pager based on CLYC. CLYC is supplied by RMDand its sister organization (Hilger Crystals).

FIG. 2 illustrates gamma and neutron capabilities of the CLYC material.It shows energy spectra measured with a CLYC crystal under moderatedAm/Be excitation. The peak at the right side of the spectrum is due tothermal neutrons. For energy calibration purposes the graph also shows a¹³⁷Cs spectrum (red curve). Based on the position of the 662 keV peak,the neutron peak appears at ˜3.2 MeV gamma equivalent energy (GEE).

These new scintillation materials have exciting properties, but they areassociated with several issues which currently increase the cost oftheir mass deployment. First issue is the production yield. RMDdemonstrated that high quality elpasolite scintillators can be grown and1″ and 2″ CLYC crystals are routinely produced and delivered. However,the yield is limited due to cracking during the cooling of the ingots,defects, inclusions, and secondary phases at the both ends of the ingot.All these result in higher cost for large size scintillators. Whileobtaining crystals with sizes above 2″×2″ is possible, currently thecost of such crystals might become prohibitive to build low costinstruments.

An additional issue with these new materials is that they are highlyhygroscopic, which complicates their handling. The standard procedure isto encapsulate them in a metal enclosure with an optical window. Whilethis process is well established and used in other “classical”scintillators like NaI:Tl or LaBr₃:Ce, it introduces additional lightloss due to the difference in the refractive index of the crystal,window and optical readout component. The encapsulation is done in dryatmosphere, but the sealing could become compromised during the years inoperation, which could lead to a degradation of the performance and longterm issues.

Recently RMD investigated novel plastic scintillators based on styreneand vinyltoluene monomers doped with wavelength shifters such as PPO foruse in applications where neutron/gamma pulse shape discrimination (PSD)is required. Measurements show that the plastic scintillators fabricatedat RMD have excellent optical quality, a good light yield (similar tocommercial plastics such as BC-404 and BC-408), and excellentgamma-neutron PSD with a Figure-of-Merit (FOM) of >3 at 2.5 MeVee gammaequivalent energy threshold. A photograph of a selection of RMD'sneutron plastic scintillators is shown in FIG. 3.

The plastic scintillators do not exhibit the same issues like the newinorganic scintillators. They have very low production cost, based onlow cost components and very high, practically 100%, production yield.They can be produced in large volumes and realized in practically anydesirable shape. Moreover, they are not hygroscopic, which significantlysimplifies their handling and reduces the cost of their utilization.

The plastic scintillators have excellent properties, but they exhibit anumber of performance limitations. First, they have low detectionefficiency due to their low density close to 1 g/cm³. Second, they donot have photopeak efficiency for energies above 30 keV due to low-Zconstituents.

SUMMARY

In one embodiment, a composite scintillator may include a scintillatormaterial encapsulated in a plastic matrix material. Depending on theembodiment, the scintillator material may include CLYC, Cs₂LiYBr₆(CLYB), Cs₂LiLaBr₆ (CLLB), Cs₂LiLa(Br_(x)Cl_(1-x))₆ (CLLBC), and otherappropriate elpasolite scinitillators. Other possible scintillatormaterials may also include SrI, LiSr₂I₅(LSI), LaBr₃, LaCl₃, NaI, and anyother appropriate scintillator material. The plastic matrix material maycorrespond to any appropriate material including, but not limited to,polystyrene (PS), polyvinylchloride (PVC) and polymethylmethacrylate(PMMA), and Polyvinyltoluene (PVT). Additionally, in some embodiments,the plastic matrix material may also be a plastic scintillator material.

In some embodiments, a composite material may have a scintillatormaterial loading of between about 50% to 75% inclusively though loadingsboth greater than and less than this range are also possible.Additionally, the scintillator material may comprise a plurality ofpieces with characteristic sizes less than about 0.5 mm, 2 mm, 5 mm, or10 mm, though other sizes are also possible. In some embodiments, alower size limit of the plurality of pieces of scintillator material maybe at least one times, or 10 times, a stopping length of alpha radiationin the scintillator material. For example, some scintillator materialsmay have a stopping length between about 10 μm to 20 μm which maycorrespond to a scintillator material size greater than 10 μm to 20 μminclusively or 100 μm to 200 μm inclusively.

It should be appreciated that the foregoing concepts, and additionalconcepts discussed below, may be arranged in any suitable combination,as the present disclosure is not limited in this respect. Further, otheradvantages and novel features of the present disclosure will becomeapparent from the following detailed description of various non-limitingembodiments when considered in conjunction with the accompanyingfigures.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures may be represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1A shows a packaged 1″ and 2″ CLYC detectors;

FIG. 1B shows a CLYC based Gamma-Neutron Pager;

FIG. 2 Energy spectra measured with CLYC under 137Cs and 241Am/Beexcitation;

FIG. 3 Photograph of a selection of RMD's plastic scintillators withdifferent shapes and compositions;

FIG. 4A is a photograph of a 1″ diameter by 1″ composite CLYC-Plasticdetectors produced in RMD's;

FIG. 4B is a sketch of a composite scintillator structure;

FIG. 5A is an energy spectra under 137Cs irradiation of 1″ diameter by1″ composite CLYC-Plastic detectors at 4 μS shaping constants and energyresolution at 662 KeV is 6.7%;

FIG. 5B is an energy spectra under neutron irradiation produced by 252Cfof 1″ diameter by 1″ composite CLYC-Plastic detectors at 4 μS shapingconstants and energy resolution of 5.3% and gamma equivalent energy of3.1 MeV of the neutron peak was measured;

FIG. 6A is an energy spectra under 137Cs irradiation of CLYC crystal(used to produce the composite detector) at 4 μS shaping constants andenergy resolution at 662 KeV is 7.8%;

FIG. 6B an energy spectra under neutron irradiation produced by 252Cf ofthe same crystal at 4 μS shaping constants and energy resolution of 8.3%and gamma equivalent energy of 3.3 MeV of the neutron peak was measured;

FIG. 7A is a pulse shape discrimination scatter plot recorded for aCLYC-plastic composite detector under 252Cf excitation;

FIG. 7B is energy spectra corresponding to gamma and neutron radiationfor the CLYC-plastic composite detector used in FIG. 7A;

FIG. 7C is a Figure of Merit (FOM) plot showing the separation of thegamma and neutron events for the CLYC-plastic composite detector used inFIG. 7A;

FIG. 8A is a photograph of a 1″ diameter by 1″ composite CLYC-Plasticdetectors with a single crystal embedded in the plastic;

FIG. 8B is a photograph of a 1″ diameter by 1″ composite CLYC-Plasticdetectors with a powdered crystal embedded in the plastic;

FIG. 9A is a photograph of 1″ diameter by 1″ composite CLYC-Plasticdetectors with a first light guide structure;

FIG. 9B is a photograph of 1″ diameter by 1″ composite CLYC-Plasticdetectors with a second light guide structure;

FIG. 10 presents energy spectra under 22Na, 241Am, 57Co and 137Csirradiation of a 1″ diameter by 1″ composite CLYC-Plastic detector;

FIG. 11 presents proportionality measurements with different radiationsources irradiating a 1″ diameter by 1″ composite CLYC-Plastic detectoras compared to NaI:Tl and LaBr3 scintillators;

FIG. 12 is a schematic representation of a multimode high resolutionradiation detector;

FIG. 13A-13B present a schematic representation of a Graphical userInterface (GUI) based on smart phone.

DETAILED DESCRIPTION

Clearly organic and inorganic scintillators are antipodes. The strengthof one of the materials is the weakness of the other. To address thatRMD is developing a novel technology which combines the positive aspectsof inorganic and organic scintillators while suppressing or eveneliminating their negative side effects. It is based on the suspensionof inorganic scintillator pieces into an organic scintillator matrix. Iffor example CLYC scintillator is used, the composite detector canprovide high gamma and photopeak efficiency, efficient thermal neutrondetection and excellent neutron—gamma discrimination. As an organicmatrix, a dual mode plastic scintillator can be used. It can providelarge gamma and fast neutron detection volume as well as a light guidefor the scintillation light generated in the inorganic material. Due tothe significant difference between the light decay of the gamma andneutron events in the plastic scintillator matrix and in the CLYCscintillator, all events can be easily discriminated using pulse-shapediscrimination.

The proposed composite scintillators provide the best features of theinorganic and organic scintillators, like excellent energy resolution,dual mode neutron—gamma operation, excellent neutron-gammadiscrimination, all that at low production cost, approaching the costfor plastic scintillators, and large detection volumes produced in anydesirable shape. In addition, this technology offers the possibility todetect fast neutrons as well, utilizing the composite detector matrix.This feature could be very useful to identify the shielding offissionable materials based on the thermal to fast neutron ratio.

Even at this early stage of the investigation, CLYC-Plastics compositedetector demonstrated better resolution compared to the classical NaI:Tlscintillators with the added benefit of efficient neutron detection andexcellent neutron-gamma discrimination. While the preliminary resultsare based on CLYC, other materials including CLLBC might be used aswell. Further, the proposed composite technology will provide detectorswith the best performance to cost ratio compared to any of the currentlyavailable scintillators.

Possible benefits of this project may include producing a very low costdual-mode detector capable of gamma ray spectroscopy with thermal andfast neutron detection. The radiation detection instrument may have thefollowing characteristics and advantages: Good gamma ray resolution(resolution better than NaI:Tl, approaching the resolution of elpasolitescintillators (5-6% with CLYC, and 4-5% with CLLBC); Good efficiency forgamma ray detection (scintillator sizes of Ø2″×10″ are visible);Efficient detection of thermal neutrons (above 80% at 2 cm compositematerial); Efficient detection of fast neutrons based the use of dualmode plastic scintillator matrix; Effective separation between gamma rayand neutron events using PSD; and Compact, simple, and low powerelectronics

The significantly lower cost of these composite detectors is based onlow cost components and higher production yield for bothconstituents—CLYC and plastics. Since the CLYC is not a solid monocrystal, but rather collection of pieces, the production yield will behigher, potentially approaching 100%. Also since the cracks are not anissue, the CLYC crystal growth can be significantly accelerated,especially reducing the long time require for the cooling of the ingot.With respect to the plastic scintillator—the cost of the materials isvery low and the production yield is closed to 100%.

The proposed system will be built around a single composite scintillator(CLYC or CLLBC in plastics), therefore, the detector design will besimpler and the detector itself more compact. So far, the detection ofdifferent types of radiation has been based on a combination ofindividual detectors, one for each type of radiation. For example,NaI(Tl) and 3He detectors have been commonly combined for gamma andthermal neutron detection, respectively. In addition, there is currentlya shortage of 3He gas, which has increased the demand for new solutionsin thermal neutron detection. The proposed detector would address thisissue as well.

This technology has the potential to be truly transformative bycombining the positive aspects of organic and inorganic scintillatorsand avoiding and even eliminating their weaknesses. The dramaticallylower production cost will provide an opportunity to build low cost,high performance radiation instruments for nuclear material control andaccountability applications. Hand-held dual gamma/neutron detectors willstrongly benefit from this technology, which will allow their massdeployment in control of nuclear material diversion and proliferation,for accountability of the materials in nuclear facilities and insafeguards applications. Large area, low cost, gamma-neutron monitoringsystems can be also realized utilizing this technology.

To prove the principle, we produced several prototypes of compositedetectors using the described technology, but instead of dual modeplastic scintillator we used standard plastic. FIG. 4A shows aphotograph of a composite CLYC-Plastics detector produced in RMD. Thesize of the detector is Ø1″×1″. The CLYC pieces were suspended in aPolymethylmethacrylate (pMMA or MMA later in the text) plastics. FIG. 4Bshows a sketch illustrating the structure of the composite detector. Itconsist of a plastic shell with a well. The CLYC pieces are placed inthe well and suspended in liquid plastics. After the polymerization,they become incorporated in the plastic body. The CLYC pieces had avolume loading of 75% in the plastic body. The plastic material of theshell and in the well has the same composition and after polymerizationthey fuse in a single plastic matrix. In this particular arrangement thescintillation elements for gamma and neutron detection are CLYC pieces,while the plastic matrix is only used to transport the light from allparts of the composite scintillator.

The experimental results confirmed our expectations for highperformance, multi-mode detectors. We obtained with Ø1″×1″ CLYC—Plasticscomposite detector a gamma resolution of 6.7% at 662 keV measured under137Cs irradiation (see FIG. 5A). The detector has a good neutrondetection capability due to the 6Li in the CLYC material. For thethermal neutron resolution we obtained 5.3% at >3 MeV gamma equivalentenergy measured under AmBe irradiation (see FIG. 5B).

The inorganic pieces for these experiments we produced by braking asingle CLYC crystal. Before braking it we evaluated itsperformance—gamma resolution and neutron detection. The reason for theseinitial measurements was to obtain information how the performancechanges due to the transition from single crystal to compositescintillator. FIGS. 6A and 6B shows the results from the initialmeasurements. Surprisingly the single crystal had slightly worse gammaand neutron resolution compared to the composite detector: 7.8% vs. 6.7and 8.3% vs. 5.3%, while both have comparable light yield. Themeasurements indicate that the original crystal was not a good qualitysample. It had a visible crack which explains the double neutron peak at3.2 and 3.4 MeV and broader gamma peak at 662 keV. One possibleexplanation to these unexpected improvement of the performance afterbraking the crystal and suspended it in plastics is that by thisarrangement the light collection from all parts of the scintillator ishomogenized. Any internal “mirrors” due to cracking are replaced withlight transparent media.

An excellent separation between the gamma and neutron events wasobserved with the new composite detector. FIG. 7A shows a PSD scatterplot recorded for a Ø1″×1″ CLYC—Plastics composite detector under 252Cfexcitation. FIG. 7B shows the energy spectra corresponding to gamma(green line), neutrons (red line), and combined (blue line) radiation.FIG. 7B shows the Figure of Merit (FOM) which illustrates the projectionof the gamma and neutron events. The plot illustrates the excellentseparation of different events which is demonstrated by a FOM of 3.0.

These results are impressive considering the early stage of thedevelopment. We obtained energy resolution for gamma and neutron eventsand neutron-gamma discrimination close to the results which can beobtained with moderate quality CLYC single crystal. While thepreliminary results are based on CLYC and plastic composition, the PhaseI study will include selection of the inorganic scintillation material.We will consider in our research other elpasolite scintillators, likeCLLBC, with the goal to find the optimal combination of organic andinorganic materials.

The proposed composite detector can provide a high performance solutionfor nuclear material accountability and safe guard applications atproduction cost approaching that of plastic scintillators. Thetechnology can deliver large detection volumes without the negativeimpact and limitations of the crystal growth process. Any shape of thedetector can be realized, which offers significant applicationflexibilities. Since the elpasolite pieces are suspended in a plasticscintillator matrix, they are protected from moisture.

Polymer Preparation

This task will involve mainly plastic matrix fabrication andinvestigation of the compatibility of the plastic and elpasolitematerials. While the ultimate goal is to use dual mode plastics toobtain fast neutron detection, in Phase I we will develop theCLYC-plastic technology utilizing plastic only as matrix to transportthe light and to be used as a moderator for fast neutrons.

The basic principle of the fabrication of plastics involves thepreparation and purification of the monomers; polymerization of themonomers followed by heat treatment and post-processing operations suchas cutting, grinding and polishing.

Purification of the Monomers

The first step in the fabrication of the organic-inorganic compositescintillators is the filtration and purification of the monomers toremove impurities, such as inhibitors, water and small particulates thatmay be present in the monomer. Removal of the inhibitor is mostimportant since it literally “inhibits” the polymerization of themonomer. Removal of the water is critical for the success of thetechnology since CLYC and other elpasolite materials are highlyhygroscopic. Excessive moisture and inclusions in the monomers willdamage the surface of the CLYC pieces and will significantly reduce thelight yield of the composite scintillator.

Moisture can be removed from the monomers by using sodium hydroxide ormolecular sieves. Alternatively, vacuum distillation can be performed.Next, a glass column is packed with activated alumina and the monomer isslowly passed through the column. The inhibitor is removed by thechromatography process, while the pure monomer is collected at thebottom of the column.

Polymerization

The second step in the fabrication of the plastic scintillators will beto determine the optimal method of polymerization: 1) thermal, 2)anionic, or 3) cationic. The interaction with the elpasolite materialwill be the criteria to select the method. The actual polymerizationwill be performed at temperatures between 60 to 140° C. for a period of1-2 weeks, depending on the size and volume of the scintillator to bemade. Thermal polymerization will be done under oxygen-free conditionsand at temperatures that do not allow the monomer to boil. Shouldbubbles appear in the monomer/polymer, the temperature will be adjusteddepending on the type and polymerization degree of the specimen. Thecontainer with the monomer will be heated evenly throughout to preventexcessive localized polymerization and to provide homogeneous heatdistribution inside the monomer. Uneven heat distribution could lead togas bubble formation and excessive laminar pressure which will preventbubbles from floating to the top and escaping the monomer/polymer. Theprogress of polymerization will be monitored daily in order to determinewhether the process has been fully completed.

Compatibility Studies

Important component of the composite technology is the compatibility ofthe plastic scintillator components and the polymerization process withthe CLYC material. We will initially experiment with CLYC scintillatorsincorporated into a range of different plastic matrices, such aspolystyrene (PS), polyvinylchloride (PVC) and polymethylmethacrylate(PMMA). The main reason for this study is that we conducted severalexperiments with CLYC incorporated into PMMA with dramatically differentoutcome. For example FIG. 8A shows a scintillator with very good opticaland spectroscopic performance. However some of scintillators were nottransparent, the light yield and resolution were poor FIG. 8B. There areseveral possibilities for these different outcomes, ranging from achange in the polymerization process to initial monomer purity, mainlythe moisture content in the initial material. These issues will beinvestigated in detail in Phase I.

Scintillator Material

The particle size is important to obtain optimal gamma and neutronefficiency and high light yield. We will experiment with different sizeof the particles. Our preliminary observations are that the performanceimproves by increasing the size of the particles. FIGS. 8A and 8B showtwo extremes: left scintillator is build using a single piece CLYCcrystal, and right—fine CLYC powder. The resolution was 6.5% and 15% at662 KeV, respectively. While the resolution on the large single CLYCpiece was comparable to the results of the scintillator with multiplepieces (see FIG. 4) at 6.6 to 6.9% in both cases, the resolution of thescintillator with fine powder was significantly worse at 15%. Inaddition, the neutron detection efficiency of the ‘powder’ scintillatorwas lower. One possible explanation is that the smaller particle willhave larger “wall effect”, i.e. the probability of the neutrons tointeract close to the surface of the particle dramatically increaseswith decreasing the size of the particles. Also reducing the size of theparticles increases the probability for light scattering andcorrespondingly light losses. More experiments are required toinvestigate these effects and to define the optimal size of theCLYC/CLLBC inclusions. Our preliminary observations are that theparticle sizes above 2-5 mm deliver more consistent light yield,resolution and neutron efficiency performance.

Load Factor, i.e. the volume ratio between the elpasolite particles andthe polymer matrix will be investigated. Our preliminary experimentswere with 50% to 75% load. The performance of 50% scintillator wasslightly better but the results were not conclusive, since thedifference could be based on other component of thetechnology—polymerization process or initial charge. A detailed studywill eliminate these variables and will provide the answer to the loadquestion. The goal will be define the maximum possible load withoutdegradation of the light yield.

Scintillator Structure, which supports the transport of the light fromany part of the composite scintillator, is another important aspect ofthe composite detectors. FIGS. 9A and 9B show two solutions—directpolymerization in the crucible FIG. 9A and polymerization in a well FIG.9B. In the well technology we produce the composite scintillator in twosteps—(1) fabricate the plastic part without any inclusions and drillmechanically a well in it, and (2) place the elpasolite particles andmonomer in the well and perform a second polymerization step tocompletely encapsulate the particles in the plastic matrix. While thetwo detectors did not show significantly different performance, webelieve that the well technology is more promising to produce very longdetectors. The shell of the initial plastic part acts as a light guidefor the light generated even in the most distant part of the compositescintillator.

Post Processing

The final step in the fabrication of the composite scintillator is postprocessing. This includes cutting, grinding and polishing of thesurfaces to obtain transparent and optically clear samples. Finally thesamples are wrapped with Teflon to provide reflective surface.

Gamma Response Evaluation

Energy resolution of 4% and 6% at 662 keV or better with inorganiccomponents CLLBC and CLYC, respectively are expected.

In one example, FIG. 10 shows spectra measured under irradiation withdifferent sources with one of our preliminary composite CLYC-Plasticscintillator (including a volume loading of 75% in the plastic body)with Ø1′×1″ dimensions. These measurements are very useful to estimatethe impact of the composition and scintillator structure on the lightyield and on the linearity of the response.

Proportionality

Using collected data, proportionality (light yield versus energy ofexcitation) of each crystal can be evaluated. FIG. 11 shows theproportionality plot of a Ø1′×1″ CLYC-Plastics composite detectormeasured with sources with different energies. The result indicate thatthe new composition has excellent linearity of 4% in 30 Kev to 1.2 MeVgamma energies, approaching the results with the other elpasolitescintillators. FIG. 11 also shows for comparison the linearity of twocommonly used scintillators—NaI:Tl and LaBr₃. The new composite detectorshows significantly better linearity, which could explain the goodenergy resolution regardless of the relatively low light yield.

Thermal Neutron Response Evaluation

Energy Resolution of the thermal neutron peak measured with Am/Be energyspectra are expected to be 6% or better.

Pulse Shape Discrimination (PSD) is based on the difference in the shapeof the neutron and gamma signals. FIG. 7A shows a PSD scatter plotrecorded for a Ø1″×1″ CLYC—Plastics composite detector under 252Cfexcitation. The plot illustrates the excellent separation of differentevents which is demonstrated by a FOM of 3.0. We will be looking tomaintain such high FOM for the prototypes.

Design and Construction of a Desk-Top Detector

We intend to implement a modular design concept, which will provideflexibility to construct the instrument and possibility to optimize theperformance of the individual modules with respect to size, powerconsumption and cost. The following modules will be evaluated for thefinal conceptual design:

Read out electronics, high voltage and associated control and monitoringmodule. It will also integrate components for temperature stabilization.

Front-end signal processing. This module will process (shape) thedetector signals, and will separate neutron and gamma events using PSDand PHD methods [Gueorguiev 12-2].

ARM based microcontroller for MCA functionality, interfaces, systemcontrol and monitoring. For the initial evaluation we plan to use anAtmel Sam3 microcontroller with an ARM core.

We will work out the concept of the user and communication interface,which will be developed and finalized in Phase II. We will consider therequirements for data storage, gamma dose rate, neutron count rate,alarm/warning indication, overload condition, instrument and batterystatus.

FIG. 12 shows the preliminary concept of the multimode high resolutionradiation detector. It has the major components listed above. We willexplore the two options of graphical user interface (GUI), an internalLCD or an external display module, e.g. a smart phone. Our preliminarytrade-off study indicates that the Atmel-Sam3 processor is the verysuitable for this application. It is low power component; has largeinternal non-volatile FLASH type memory for the program and largeinternal RAM memory to build the MCA spectra. In addition, it offers anumber of built-in interfaces—LCD, USB, UART, Ethernet, I2C, 16-channelADC and 2-channel DAC. It consumes only a <10 mA current which willprovide for long battery life.

The standard serial port (UART—Universal AsynchronousTransmitter/Receiver) will be used to communicate with a Bluetoothmodule, which will provide connectivity to an external mobile device,e.g. a Smart Phone or iPad. The USB port will be used to communicatewith a PC or Laptop. The USB 5V power line will be used to charge theinternal rechargeable batteries.

FIGS. 13A and 13B shows the preliminary design concept based on a smartphone or iPad as an external display option. The signal processing andMCA will be performed in the radiation detector. An external smart phoneis used as a convenient display. The communication between the radiationdetector and the smart phone will be provided through the Bluetoothwireless interface. The details of the concept and the correspondingapplication for the smart phone will be developed and demonstratedduring Phase II.

Example: Gamma Ray Spectroscopy Comparison

Scintillation crystals coupled to photomultiplier tubes are the mostcommon detectors for gamma ray spectroscopy. The choice of scintillatordepends on the requirements of the application. Some of the requirementsinclude high stopping efficiency, fast response, high light output, goodproportionality, minimal afterglow, low cost, good environmentalstability, and, of course, good energy resolution. None of thecommercially available inorganic scintillators can satisfy all of theserequirements. Table I provides a comparison of the common inorganicscintillators used for gamma ray detection and spectroscopy. For manyapplications, NaI:Tl is the scintillator of choice. NaI:Tl has highlight output, moderate speed, and its emission is well matched tophotomultiplier tubes. However, its energy resolution is poor, about6-7% FWHM at 662 keV. Relatively poor energy resolution is observed inthe majority of scintillators developed to date, as seen in the table,which arises from the poor proportionality of these materials,particularly at low energies.

Table I also lists the scintillation properties of CLYC and CLLBC. Ourstudies indicate that both exhibits excellent proportionality over awider energy range, even better than LaBr₃:Ce. As a result, the energyresolution achieved with CLLBC is comparable to that of LaBr₃:Ce, 3% at662 keV, despite its lower light yield than LaBr₃:Ce. Another advantageof CLLBC over LaBr₃:Ce is its isotropic cubic elpasolite structure,which implies that the growth of large volume crystals of CLLBC shouldbe easier compared to that of hexagonal compositions. This has beenconfirmed in our research with another elpasolite, CLYC.

The table also shows the preliminary data for the composite CLYC-Plasticscintillator. The density of 2.7 g/cm3 is based on 75% volume load ofCLYC in plastics, which number corresponds to the load which we used inour preliminary experiments. The resolution and light yield are alsofrom our preliminary experiments and we believe that it can be improvedby optimization of the technology.

Due to the high light yield, CLLBC could be potentially better candidatefor the composite detector. In Phase I we will investigate if thetechnology, currently prototyped only with CLYC scintillators, will besuitable to CLLBC as well. Since both scintillators are elpasolite weexpect that both material can be used to build composite detectors.

TABLE I λ, LY, Decay time, En. Res. @ Density, Scintillator nm ph/MeV ns662 keV, % g/cm³ NaI(Tl) 415 38,000 230 6-7 3.67 CsI(Tl) 540 52,000 10006-7 4.5 BGO 505  8,200 300 8 7.13 LaCl₃:Ce 330 49,000 25,210 3.5 3.8LaBr₃:5% Ce 360 65,000 16,330 3 5.1 CLYC:Ce 370 20,000 1,60,900 4 3.3CLLBC:Ce 410 40,000 55,270 3 ~4.2 CLYC + Plastics 370 18,000 1,60,9006-7 ~2.7

Example: Neutron Detection

In addition to gamma radiation, neutron detection can provide importantinformation for the identification of fissionable material. The neutronsare usually divided into three categories according to their energy:cold (<25 meV), thermal (˜25 meV to 1 eV), and fast neutrons (>1 eV).

Because neutrons are electrically neutral, their detection is difficult.However, some isotopes such as ³He, ⁶Li, ¹⁰B, and ¹⁵⁷Gd have large crosssections for thermal neutron absorption, and useful neutron detectorscan be developed by incorporating these isotopes as constituents. Eachabsorption process (followed by a reaction) is characterized by: crosssection, Q value, and reaction products. Table II lists these variablesfor ⁶Li and ³He isotopes [Convert].

Critical for our purposes, the ⁶Li interaction produces chargedparticles (an alpha particle and a triton with total energy of 4.78 MeV)that are readily absorbed in the scintillator, producing a bright flash.Despite a modest cross-section of ⁶Li for thermal neutron capture,usually less than 1 cm of ⁶Li material is enough to absorb the majorityof neutrons (˜80%). This characteristics, and the additional fact thatsome Li based scintillators allow for gamma neutron discrimination, makethem an excellent choice for thermal neutron detection.

A comparison of intrinsic neutron scintillators incorporating Li ions isprovided in Table III. CLLBC shows light yield of 120,000photons/neutron. Only LiF/ZnS:Ag phosphor has comparable light yield,which in thick specimens is degraded due to a light absorption.

It is important to note that multi-mode detection requires ascintillator to both register and distinguish between different types ofradiation. Among the listed materials, clearly CLYC and CLLBC are thebest candidates, with CLLBC having an advantage of better energyresolution.

In addition to scintillator detectors, there are two other main classesof thermal neutron detectors: semiconductor based and ³He tubes. None ofthese detectors are capable of detecting gamma and neutron particlessimultaneously, making them unsuitable for dual mode operation.

The presence of a plastic scintillation matrix required to incorporateinorganic material can be utilized to detect fast neutrons. A briefdiscussion of the fast neutron detection methods follows.

As discussed in the previous section neutrons with energies >1 eV areconsidered fast. At these energies the detection of neutrons is based onthe scattering of neutrons on hydrogen isotopes. Hydrogen is thepreferred nuclide to use because a colliding neutron can transfer partor all of its kinetic energy to the proton in one collision. Thisimplies that the average recoil proton energy will be half the initialneutron energy. Hydrogen has a cross section of about 20 barns forneutrons in the energy range between 1 eV and 10 keV. From 10 keV up to10 MeV the neutron cross section decreases to about 1 barn, however itis still the highest of all isotopes.

Plastic scintillators are readily available in large sizes at low costand are very safe to handle. The main limitation of commerciallyavailable plastic scintillators such as BC him him-408 is theirinability to provide efficient neutron/gamma PSD. Fortunately, recentresearch at RMD and LLNL has shown that large plastic scintillatorsbased on polystyrene and polyvinyltoluene, respectively, doped with PPOcan provide effective neutron/gamma PSD. As shown in Table IV, RMD'splastic scintillators have a high light output (higher than BGO), a fastscintillation decay and exhibit excellent neutron/gamma PSD.

TABLE II Reaction Q value σ_(th) (barns) ³He ³ ₂He + ¹ ₀n → ₁ ³H + ₁ ¹p0.764 5330 ⁶Li ⁶ ₃Li + ¹ ₀n → ₁ ³H + ₂ ⁴α 4.78 940

TABLE III decay Light yield photons per GEE, time, density, Materialneutron 1 MeV γ-ray MeV ns g/cm³ PSD Li-Glass 6,000 4,000 1.5 75 2.5 NoLiF/ZnS:Ag 160,000 50,000 — 1,000 2.6 N/Y Lil:Eu 15,000 52,000 3.5 1,2004.08 No CLYC:Ce 70,000 20,000 3.3 1^(CVL); 3.3 Yes 60; 900 CLLBC:Ce120,000 40,000 3.2 55, 270 ~4.2 Yes CLYC + 60,000 18,000 3.1 55, 270~2.5 Yes Plastics

TABLE IV Light Decay λ, yield time Scintillator nm ph/MeV ns PSD BC-408plastic 425 12,800 2.1 poor BC-501A liquid 425 15,600 3.2 very goodAnthracene 445 20,000 30 poor Stilbene 380 14,000 3.5 good RMD Plastic450 13,000 <10 very good

While the present teachings have been described in conjunction withvarious embodiments and examples, it is not intended that the presentteachings be limited to such embodiments or examples. On the contrary,the present teachings encompass various alternatives, modifications, andequivalents, as will be appreciated by those of skill in the art.Accordingly, the foregoing description and drawings are by way ofexample only.

What is claimed is:
 1. A composite scintillator comprising: a plasticmatrix material selected from the group consisting of polystyrene,polyvinyl chloride and polymethylmethacrylate; and particles comprisinga scintillator material comprising Cs₂LiYCl₆ (CLYC) encapsulated in theplastic matrix material, the particles having sizes greater than 2-5 mm,wherein the composite scintillator includes a loading of 50-75% of theparticles by volume.